The present invention generally relates to a tomographic apparatus and method for constructing a two-dimensional image of an object slice from linear projections of ionizing radiation, particularly X-rays, that is transmitted through the slice.
The invention is usable in a variety of fields including medical radiology, computerized tomography (CT), microscopy, and non-destructive testing.
It is of great importance in many fields of technology to be capable of constructing a two-dimensional illustrative representation from a series of linear data resulting from various projections (i.e. line-of-sight measurements) taken in the plane of a two-dimensional planar slice or cut of the object one desires to reconstruct. For instance, by employing X-rays to provide a two-dimensional image of a human brain it is commonly known to pass, in the plane of the cut, a planar beam of X-rays through the head and to measure the absorption of X-rays for a number of different directions.
Passing a planar beam of radiation through an object and detecting the amount of absorption within a cut of the object results in a two-dimensional object being projected onto a one-dimensional image. Similarly, passing a planar beam of radiation through an object and detecting the amount of absorption within the object results in a three-dimensional object being projected onto a two-dimensional image. This results inevitably in superimposition of information and resulting loss of the information. Complex techniques have thus to be employed if one wishes to perform an examination with greater sensitivity to spatial variations in radiation absorption and less severe superimposition effects.
In an examination method known as computerized tomography a source of a planar radiation beam and a detector (photographic film or digital detector) are arranged for irradiating the object, e.g. a human head, to be examined by the planar beam, thus defining a thin cut through the object, and for detecting the amount of radiation passed trough (i.e. not absorbed or scattered off) the object. The radiation source and the detector are revolved along a circular or other path around the object in the plane of the planar radiation beam and measurements are performed at several positions, e.g. at every fifth degree of revolution. A two-dimensional reconstruction process of the thin cut of the object is then performed, wherein e.g. brain tissue, bone, liquid-filled cavities, delimited cerebral haemorrhage, if any, etc. become distinguishable as these structures show different absorption. The process may then be repeated for each cut desired to be imaged.
Computerized tomography is described in e.g. U.S. Pat. Nos. 5,414,622; 5,398,684; 5,099,846; 5,391,877; 5,414,622 and 5,402,462.
Means for detecting the radiation in tomographic apparatus of the kind depicted above include X-ray film, various kinds of scintillator-based detectors, solid-state detectors and gaseous detectors. Gaseous detectors are very attractive for this purpose, particularly at lower photon energies. The main advantage of these detectors is that they are cheap to manufacture compared to solid-state detectors, still providing digitized signals.
Tomographic apparatus including gaseous detectors for the detection of X-rays are described in e.g. U.S. Pat. Nos. 4,719,354 and 4,707,607.
Major drawbacks of such tomographic apparatus employing gaseous detectors are, however, the relatively low spatial resolutions that are obtained, and that the sensitivities are somewhat limited due to short ionizations lengths and/or unwanted influences from fluorescent X-ray photons. Further drawbacks include small signal amplitudes and fairly high noise. Also, these tomographic apparatus involves a high dose to the patient.
Accordingly, it is an object of the present invention to provide a tomographic apparatus and method for constructing a two-dimensional image of an object cut from linear projections of ionizing radiation, particularly X-rays, that is transmitted through said cut, which provide for an improved spatial resolution.
A further object of the invention is to provide such tomographic apparatus and method, which are very sensitive and can thus operate at very low X-ray fluxes.
Still a further object of the present invention is to provide such tomographic apparatus and method, which are effective, fast, accurate, reliable, easy to use, and of low cost.
Yet a further object of the invention is to provide such tomographic apparatus and method, which employ a detector that can be given a length, in the direction of the incoming radiation, for achieving a desired stopping power, which makes it possible to detect a major portion of the incoming radiation.
Still a further object of the invention is to provide such tomographic apparatus and method, which employ any kind of ionizing radiation, including electromagnetic radiation as well as incident particles.
These objects among others are, according to the present invention, attained by a tomographic apparatus and method as claimed in the appended Claims.
In this respect a particular feature of the invention is that it employs a detector, in which electrons released by interactions between photons and the ionizable substance can be extracted in a direction essentially perpendicular to the incident radiation. Hereby it is possible to obtain an improved spatial resolution.
Further, the gap between the electrodes of the detector may be adapted (small enough) to prevent the majority of produced fluorescent X-rays to affect the detected signals, whereby yet a better spatial resolution is obtained.
By employing avalanche amplification of electrons released from ionization (through primary and secondary reactions) in the detector a particularly sensitive tomographic apparatus and method are achieved, which provide for the employment of extremely low doses of radiation, still obtaining signal levels high enough for construction of a two-dimensional image of an object cut exhibiting very low noise levels.
Further characteristics of the invention and advantages thereof will be evident from the following detailed description of preferred embodiments of the invention, which are shown in the accompanying drawings.